One process for uranium isotope separation requires a source of uranium vapor. This vapor can be supplied by an electron beam evaporator. U.S. Pat. No. 4,035,574 described such an evaporator. The uranium evaporation takes place within a vacuum chamber. During the evaporation process the activity within the chamber is quite violent and the environment inside the chamber is quite hostile. Temperatures sufficient to vaporize uranium exist within the chamber. Uranium liquid and vapor, both present, are extremely corrosive.
During the isotope separation process the pool of molten uranium from which the vapor originates becomes depleted. It is desirable to maintain the level of the pool within certain limits. Control over feeding of the uranium stock to replenish the pool can be performed manually. Manual control requires the operator to view the level of the pool through a window in the chamber. The visibility required for this control method is tenuous, due to splattering of the window and violent activity within the chamber. It is desirable to implement a level control system less sensitive to conditions in the chamber and not requiring manual intervention.
In order to implement such an automatic level control system, a means of measuring the level of the molten metal pool is required. One method has been to monitor the weight of the crucible. For this method force transducers are located inside the hostile environment of the chamber. As splattering may cause build up of metal outside the molten region, the level of the molten region of the pool can drop without being sensed as a weight change. Another method requires an active probe beam such as supplied by a helium-neon laser. The probe beam is reflected off the surface of the metal pool and picked up by a detector. The laser and detector can be located outside the chamber, but two optical ports into the chamber are required. During evaporation the violent activity makes reception of a reflected beam unreliable.